Presently under development is the use of magnetic tape as the recording media for digital cassette recording for use in the electrical component and computer industries. Digital cassette recording requires vast amounts of magnetic tape. As such, reliable and efficient apparatus for certification of production magnetic tape is necessary to ensure the required quality and quantity of tape.
A magnetic tape cassette for digital recording consists of a plastic cartridge with concentric supply and take-up reels, with approximately 1000 ft. of magnetic tape per cassette, having a capacity for approximately 930 ft. of tape for recorded data.
Digital cassette recorders utilize a helical scan technique for recording wideband data and a longitudinal recording technique to record positioning data onto the magnetic tape. The wideband data is serially recorded on the magnetic tape at a 14.degree. angle to the edge of the tape and has a packing density of 45 Kbits per inch. Spacing between adjacent scans is approximately 0.03 millimeters, thus, a small tape defect can introduce large unacceptable data errors that require costly and time consuming correction.
Additionally, since digital cassette recording "writes" data at an angle to the tape edge, it is much more sensitive to longitudinal scratches which do not normally effect longitudinal recording.
Heretofore, there have been three basic concepts in use for certification of magnetic tape. These include laser scanning, helical scan record/playback and longitudinal record/playback Each are briefly discussed below.
Laser scanning consists of using a laser to scan the tape for tape defects. It is a useful quality control tool for analyzing tape surface and edge conditions. Laser scanning does not provide, however, any information about the magnetic characteristics of the tape, or the performance of the tape when used in a digital cassette recorder.
The helical scan record/playback certifier utilizes the helical scan approach for the recording of test data and reproduction of the same data to verify tape quality. It provides an excellent test in an environment very similar to a digital cassette recorder configuration. One disadvantage of the helical scan record/playback certifier is that two tape passes are required (record followed by playback), which is very time consuming. Additionally, the certifier must track very accurately, otherwise, any mistracking would be interpreted as a tape defect.
The longitudinal record/playback certifier uses a standard longitudinal transport to record and reproduce data. This can be done at high speeds, approximately 100 inches per second, with a single tape pass (read after write operation), thus, yielding a high output for tape certification. This method, however, not only does not detect, but can cause longitudinal irregularities, such as scratches. The improved signal to noise ratio from the wider track widths of a longitudinal system can also mask certain tape problems.